1. Field of the Invention
The present invention relates to a control apparatus for a spark ignition type direct injection engine, which can remove nitrogen oxides (NOx) in exhaust gas even at lean air-fuel ratios by inserting an NOx adsorbing catalyst that adsorbs NOx in exhaust gas in an excess oxygen atmosphere into the exhaust path of the engine.
2. Description of the Related Art
Conventionally, as for a control apparatus for an engine of this type, the following control technique is known. That is, the air-fuel ratio of a combustion chamber is controlled to be a predetermined lean state (e.g., A/F.gtoreq.20) to improve fuel economy, and NOx in exhaust gas that has become an excess oxygen atmosphere at that time is adsorbed and removed by an NOx adsorbing catalyst. Before the adsorbed NOx amount of that NOx adsorbing catalyst becomes excessive and the adsorption performance drops, the air-fuel ratio of the combustion chamber of the engine is switched to a rich one substantially equal to or higher than the stoichiometric air-fuel ratio, thereby releasing NOx from the NOx adsorbing catalyst (to be also referred to as refresh or regeneration of the catalyst).
The technique disclosed in Japanese Patent Laid-Open No. 10-274085 exploits certain characteristics of the aforementioned NOx adsorbing catalyst; it oxidizes NOx in exhaust gas in an excess oxygen atmosphere and adsorbs it in the forms of nitrates, and releases NOx by substituting the adsorbed nitrates by carbon monoxide (CO) in exhaust gas when the oxygen concentration decreases. That is, the oxygen concentration in exhaust gas is decreased upon releasing NOx from the NOx adsorbing catalyst, and the CO concentration in exhaust gas is increased by injecting and re-combusting additional fuel in an expansion or exhaust stroke of each cylinder, thus promoting release and deoxidization/purification of NOx.
Upon releasing NOx from the NOx adsorbing catalyst, the air-fuel ratio of the combustion chamber is normally controlled to stay rich (A/F=12 to 13) so as to decrease the oxygen concentration, and to increase the concentrations of reducible, uncombusted hydrocarbons (HC) and CO. In this manner, release of NOx from the NOx adsorbing catalyst can be promoted, as well as deoxidization/purification of NOx. On the other hand, since extra fuel is consumed to only refresh the catalyst, fuel economy suffers.
In the conventional control apparatus (Japanese Patent Laid-Open No. 10-274085) that injects additional fuel in the expansion or exhaust stroke of each cylinder, the oxygen concentration in exhaust gas can be decreased and the HC and CO concentrations can be increased without any engine output drift. However, since the additional fuel has nearly no contribution to the engine output, fuel economy may deteriorate considerably.